1. Field of the Invention
The present invention relates to an optical fiber cable having improved compatibility with a waterblocking filling composition. In particular, the present invention relates to an optical fiber cable comprising a cable component, wherein an optical fiber can be housed, such as a buffer tube or a slotted core, made of a polyolefinic material having high compatibility with a waterblocking filler disposed therein.
2. Description of the Related Art
Examples of structures of optical fiber cables known in the art are described for instance in U.S. Pat. No. 5,911,023 and comprise the multi-tube (also know as xe2x80x9cloose tubesxe2x80x9d) structure, the monotube (or central loose tube) structure and the slotted core design structure.
According to the multi-tube structure, a number of buffer tubes containing one or more optical fiber (either as single fibers or in the form of bundles or ribbons of fibers) are disposed around a central element, which may be a strength member (e.g. made of fiberglass or polymeric coated steel wire) or a further buffer tube. Optical fibers or optical fiber ribbons are preferably loosely housed within the buffer tube, so to minimize stresses caused by elongation of the cable structure. The buffer tubes are tipically helical stranded around the central element with a continuous or with a S-Z (alternate hand) helix. One or more buffer tubes may be replaced in the configuration by one or more rods (typically of plastic material) to provide symmetry for fiber counts lower than that of a full fiber count cable. The buffer tubes are generally filled with a waterblocking material such as a gel or grease, which surrounds the optical fibers and prevents longitudinal propagation of water along the tube.
According to the central loose tube design, the optical fibers are disposed within a central polymeric tube which is generally filled with some type of waterblocking compound. In all of these structures, the buffer tube or core provides the primary structure to protect the thin optical fibers contained within.
According to the slotted-core design, a number of channels or slots forming a helical path are provided on the outer surface of polymeric rod centrally disposed within the cable structure. The optical fibers are disposed within such channels or slots which are generally filled with a waterblocking gel.
Typically the buffer tubes or core is jacketed with an additional protective layer.
Additionally reinforcing yarns or fibers as well as waterblocking materials in the form of gels or hot melts, water swellable powders, yarns, or tapes, and/or corrugated armor may be placed between the jacket and the inner cable layers.
As disclosed in U.S. Pat. No. 5,911,023, fiber optic buffer tubes or cores have been primarily made from xe2x80x9cengineering resinsxe2x80x9d such as polybutylene terepthalate (PBT), polycarbonate (PC), a polyamide such as nylon-12, or some layered combination of the above. Generally, these materials have been chosen due to their high modulus and low CTE relative to other polymers.
In addition, U.S. Pat. No. 4,153,332 suggests using polyethylene or polypropylene as a material suitable for the manufacturing of stranded loose buffer tubes.
Furthermore, U.S. Pat. No. 5,574,816 suggests the use of polyolefin buffer tubes made of a nucleated copolymer of polyethylene and polypropylene.
As disclosed in U.S. Pat. No. 5,911,023, such nucleated copolymer of polyethylene and polypropylene should however possess a high melt flow index (MFI higher than about 3 g/10 min) in order to increase its processability at high speed line. According to the above cited patent, the presence of such nucleating agent results in a reduced post-extrusion shrinkage of the buffer tube and allows a more rapid development of a higher level of cristallinity within the polymer.
The Applicant has now observed that the compatibility of a cable component with the waterblocking filler can be substantially improved if said cable component is manufactured by employing a suitable polyolefinic material, polyethylene in particular, having a density in the finished component of at least 0.940 g/ml, preferably of about 0.942 g/ml or higher, up to about, e.g., 0.975 g/ml.
Applicant has found that such density can be obtained by using starting materials having a nominal density (i.e. as given on the data sheet of the material) of at least 0.950 g/ml or higher.
The Applicant has further observed that if such material is polyethylene having a nominal density higher than 0.950 g/ml, said material is capable of developing a relatively high cristallinity degree (higher than 60%), also if rapidly cooled from its molten state to ambient temperature (e.g. within 10 seconds), without any substantial addition of nucleating agent.
On the contrary, other polyolefins, such as polypropylene or copolimers ethylene-propylene, need the presence of a nucleating agent (e.g from 0.05 percent to 1 percent by weight, as mentioned in the above cited EP 890,860) in order to reach a sufficient cristallinity degree in such relatively short time.
The applicant has observed that with the polyethylenic materials of the present invention, the presence of such nucleating agent in the above amounts does not substantially increase the cristallinity degree of the extruded polymer.
It will be appreciated by those skilled in the art that by avoiding such nucleating agent into polymeric compositions, the manufacturing process of the cable component is rather simpler. As a matter of fact, if such nucleating agent is used, it should be very well dispersed into the polymeric matrix. However, in order to reach an acceptable degree of dispersion, it is not possible to introduce the nucleating agent and the polymer directly into the extruder as such, but a pre-mix of the two components should be separately prepared in advance. It can thus be appreciated that the above procedure disadvantageously introduces a further step in the manufacturing process.
In addition, the Applicant has observed that in order to further select suitable polyolefinic materials, particular attention should be paid to the melt flow index (MFI) of the material and to the shear sensitivity of the same, i.e. the ratio between the MFI measured at 190xc2x0 C. and 21.6 kg and the MFI measured at 190xc2x0 C. and 2.16 kg, according to ASTM method D1238.
One aspect of the present invention thus relates to an optical cable comprising a cable component made of extruded polyolefin material wherein the polyolefin material forming said component has a density of at least 0.940 g/ml or higher, preferably of at least 0.942 g/ml or higher. Preferably, said cable component is in contact with a waterblocking filling composition, said waterblocking filling composition being preferably a polyolefin oil based composition.
According to a preferred aspect, said cable component is made from a polyolefin material having a melt flow index at 190xc2x0 C. and 2.16 kg lower than about 3 g/10 min, preferably lower than about 2 g/10 min.
According to a further preferred aspect, the ratio between the melt flow index at 190xc2x0 C. and 21.6 kg and the melt flow index at 190xc2x0 C. and 2.16 kg of said polyolefin material is higher than about 40, preferably higher than about 70.
Preferably, said polyolefin material is polyethylene.
In the present description, the term polyethylene is intended to comprise either homopolymers obtained by polymerization of ethylene monomer or copolymers obtained by copolymerization of ethylene with minor amounts (e.g. less than about 5% by mole with respect to the amount of monomers) of other unsaturated monomers, such as olefins (e.g. propylene, butene, isoprene, hexene), styrene, vinylacetate, ethylacrylate.
According to a preferred embodiment, said cable component is in contact with a waterblocking filling composition. In particular, said cable component can be a buffer tube comprising at least one optical fiber housed therein or a slotted core comprising at least one groove extending longitudinally along the outer surface of said core and at least one optical fibre housed within said groove, said at least one optical fiber being preferably immersed into a waterblocking filling composition in contact with said cable component.
According to a preferred embodiment, said waterblocking filling composition contacted with said polyolefin cable component comprises a polyolefin base oil, said polyolefin base oil being preferably an internal polyolefin.
According to a preferred aspect of the present invention, the polyolefin material forming said cable component has a density of 0.950 g/ml or higher when subjected to a natural cooling from its molten state.
Preferably said polyolefin material shows a solubility in n-heptane at 85xc2x0 C. after 24 hours of less than about 2%.
Preferably, said polyolefin material shows a weight increase of less than 10%, preferably of less than about 8% when contacted at 85xc2x0 C. with a polyolefinic oil having a kinematic viscosity of about 30 cSt at 40xc2x0 C., a viscosity index of about 128 and a weight average molecular weight in the range from approximately 400 Dalton to approximately 600 Dalton.